Apache, its not just women. My bf's moods change like the weather in Texas. Some days he can flip out if I ask him how his day was.

On the whole, for women it is hormones....the only time I personally ever really get moody is 'that time of the month'. But just like guys, we can snap due to fatigue or stress. Is there really any way to determine if your partner is fatigued or stressed beforehand? Sometimes yes, sometimes no.